If you need strong security, then any Google Pixel starting from Pixel 8 (The Pixel 8's chip includes ARMv9 architecture, which supports MTE and Branch Target Indicators (BTI) for enhanced memory safety that provides protection from entire class of exploits). Get one with unlocked bootloader (available starting from $250 on ebay). Flash the Graphene OS - not really "fun" but the most secure option if that is what you need.

Otherwise, for purely "fun" option, I would get smth like Google Pixel 3a (about $150) and install Ubuntu Touch on it https://devices.ubuntu-touch.io/device/sargo/ .

The most practical option is probably to get Google Pixel 9/9A and use it as is with the latest Android, Google AI, etc. Also having an option to flash something else (like Graphene OS) if needed

Pick something recent that supports GrapheneOS, or wait a bit as there are some rumors that they're working with a manufacturer

https://grapheneos.org/faq#supported-devices